BISAT : THE MILLSTONE GRIT SEQUENCE 
21 
Using the two well established fossiliferous horizons to split 
up the series, we obtain the following groups of beds : — 
Brimham Grit Group 350 ft. 
Cayton Gill Beds 150 ft. 
Barren Beds 250 ft. 
Colsterdale Fossil Beds 10 ft.-80 ft. 
Basement Shales say 300 ft. -400 ft. 
Basement or Bearing Grit 0 ft.-200 ft. 
The thicknesses given to the first three sections are of the series 
as developed at Glints, near Leighton Reservoir, Masham ; the thick- 
ness of the Colsterdale fossiliferous horizon (Plate II.) appears to vary 
to the indicated extent ; and the thickness of the basement shales is 
estimated from the Coverdale exposures. 
These basement shales, which together with the basement or 
Bearing Grit form the lowest beds in the series, lie on a variable base, 
which is seen in Nidderdale and Coverdale ; and indications of erosion, 
which may be responsible for a good deal of the variation, are not 
lacking. The Bearing Grit, above alluded to, is seen very well in 
Howstean Beck and Blayshaw Gill near Lofthouse-in-Nidderdale, 
and appears to attain a maximum thickness along a north and south 
line under Great Whernside. The shales are usually fairly sandy, 
and contain some small sandstone bands, but are generally of a limp 
character. They are to some extent micaceous, and thin coals occur 
near the base. Occasional small obscure shells are met with, but I 
have not yet had the opportunity of examining the whole of the thick- 
ness, and a good deal of work remains to be done in the examination 
of these shales, and the underlying junction with the Yoredale lime- 
stones. 
At the top of the above shales a sandstone or grit band occurs 
almost everywhere, with a coal seam in part of the area. This coal 
seam has been worked in Coverdale, Nidderdale and Colsterdale, 
has a thickness varying up to 1 ft. 6 in. or 2 ft., and is a useful guide 
to horizon. It is followed by shales containing a marine fauna. These 
fossiliferous shales may be traced from Colsterdale to Great Whernside 
and southwards from thence along the borders of Nidderdale and 
Wharf edale. Along this latter line the thickness of fossiliferous 
shale does not appear to be more than 10 ft., including a thin lime- 
stone band about a foot in thickness, which has been mapped and named 
by the Survey the " Tesselated Limestone " ; but the fossiliferous 
